Synthetic photometry of OB star clusters with stochastically sampled IMFs: analysis of models and HST observations

Abstract

We present a pilot library of synthetic NUV, U, B, V, and I photometry of star clusters with stochastically sampled IMFs and ionized gas for initial masses, Mi = 103, 104, and 105 M⊙; t = 1, 3, 4, and 8 Myr; Z = 0.014 and Z = 0.002; and log(US) = −2 and −3. We compare the library with predictions from deterministic models and observations of isolated low-mass (<104 M⊙) star clusters with co-spatial compact H ii regions. The clusters are located in NGC 7793, one of the nearest galaxies observed as part of the HST LEGUS and Hα-LEGUS surveys. (1) For model magnitudes that only account for the stars: (a) the residual |deterministic mag - median stochastic mag| can be ≥0.5 mag, even for Mi = 105 M⊙; and (b) the largest spread in stochastic magnitudes occurs when Wolf-Rayet stars are present. (2) For Mi = 105 M⊙: (a) the median stochastic mag with gas can be >1.0 mag more luminous than the median stochastic magnitude without gas; and (b) nebular emission lines can contribute with $>50{{\ \rm per\ cent}}$ and $>30{{\ \rm per\ cent}}$ to the total emission in the V and I bands, respectively. (3) Age-dating OB-star clusters via deterministic tracks in the U-B versus V-I plane is highly uncertain at Z = 0.014 for Mi ∼ 103 M⊙ and Z = 0.002 for Mi ∼ 103–105 M⊙. (4) For low-mass clusters, the V-band extinction derived with stochastic models significantly depends on the value of log(US). (5) The youngest clusters tend to have higher extinction. (6) The majority of clusters have multi-peaked age PDFs. (7) Finally, we discuss the importance of characterizing the true variance in the number of stars per mass bin in nature.

H$α$ Morphologies of Star Clusters in 16 LEGUS Galaxies: Constraints on HII region evolution timescales

“…As in (Hannon et al 2019), we also observe manifestations of stochastic sampling of the IMF. Most notably, the positions of our clusters in UBVI space are consistent with those predicted by the stochastic modeling of low-mass clusters by Fouesneau & Lançon (2010), Fouesneau et al (2012), and also Orozco-Duarte et al (2022). Figure 2 of Fouesneau et al (2012) provides an example of these predicted positions in the same color space as Figure 6, highlighted by a spray of points to the lower-right of the 10 Myr point of the model SSP track.…”

“…With a median cluster mass of ∼1000 M , we find that the reddening of clusters without H𝛼 are likely overestimated due to the presence of bright red stars, a result of stochastic sampling of the IMF. We also see clusters with concentrated H𝛼 which are found blueward of the young end of the SSP models, potentially due to additional contributions from nebular emission (Zackrisson et al 2001), or the presence of two or more Wolf-Rayet stars (type WC or WNE; see Orozco-Duarte et al 2022). These latter stochastic sampling effects instead have a bias toward bluer colors, and thus lower E(B-V).…”

SDSS-IV MaNGA: pyPipe3D Analysis Release for 10,000 Galaxies

“…Charlot and Bruzual (C&B) models). Even though the C&B models have been used by several authors (e.g., Mayya et al 2020;Werle et al 2020;González Delgado et al 2021;Senchyna et al 2021;Orozco-Duarte et al 2022;Senchyna et al 2022;Werle et al 2022), a detailed description of the ingredients of these models is lacking in the literature. For completeness, and for the benefit of the reader, we summarize in this appendix the characteristics of these C&B models.…”

Ionization and Star Formation in the Giant HII Region SMC-N66

“…We do not propagate the effect of a potential ∼5% uncertainty in distance in our distance-dependent estimates. Fumagalli et al 2011;Orozco-Duarte et al 2022). The situation is further complicated when binary evolution is included (Eldridge 2012).…”